The present invention relates to an air-fuel ratio control apparatus utilized for an internal combustion engine.
There is known an air-fuel ratio feedback control system (closed-loop air-fuel ratio control system) for compensating the mass ratio of air and fuel in an internal combustion engine by controlling the additional air or fuel supplied to the engine in accordance with a detection signal from an air-fuel ratio sensor. The air-fuel ratio sensor detects the actual air-fuel ratio condition of the engine by detecting whether the concentration value of a predetermined component, for example, the oxygen component, in the exhaust gas is greater than or less than a predetermined value. In an internal combustion engine having such a conventional closed-loop air-fuel ratio control system and also having an open-loop air-fuel ratio control system, for example, a carburetor or an open-loop controlled fuel injection system, if the air-fuel ratio controlled by the open-loop system deviates from the correct one, or if a variable of the air-fuel ratio which is controlled according to the closed-loop system always has a fixed deviation, the average value of the final controlled air-fuel ratio of the engine (hereinafter called the average air-fuel ratio) deviates from a desired air-fuel ratio, in spite of the closed-loop control. This is because, according to the conventional closed-loop control system, the air-fuel ratio is not controlled by detecting the deviation value of the actual air-fuel ratio from a desired value, but is controlled by detecting whether the actual air-fuel ratio is greater than or less than a desired value. If the average air-fuel ratio deviates from the desired value, the purifying efficiency of a catalytic converter for reducing the noxious components in the exhaust gas will significantly decrease.